Eosinophils are frequently associated with allergic and parasitic disease. Cellular interactions in these conditions may be crucial in host defense and immunity, as well as the expression of clinical disease. Do eosinophils and lymphocytes modulate each other's function? The effect of normal (unstimulated and mitogen stimulated) lymphocytes and lymphocytes from hypereosinophilic patients on selected eosinophil functions will test the hypothesis that lymphocytes/lymphokines moudlate eosinophil ufnction. Superoxide anion production and hydrogen peroxide generation will be examined to see whether lymphocytes can activate, augment or inhibit the oxidative metabolism of resting eosinophils. Activation, inhibition, or augmentation of degranulation will be studied using lysosomal enzyme release. Eosinophil peroxidase system activity as measured by iodination will be stuided in the presence of lymphocytes and lymphokines. The question of whether eosinophils can activate lymphocytes will be addressed by measuring possible effects of purified eosinophils, eosinophil extracts, and eosinophil granules on T lymphocyte functions (antigen and mitogen induced proliferation, lympholine production, supressor cell function) and B lymphocyte function (in vitro IgE and IgG synthesis). These studies should provide additional insight into the pathophysiology of parasitic disease, hypersensitivity reactions and atopic disease.